Muscle contraction

Definitions & Key takeaways

Muscle contraction is the process by which muscle fibers shorten to generate force. Muscle cells contain proteins called actin and myosin that interact with one another and form cross-bridges to produce a contraction. The termination of muscle contraction is followed by muscle relaxation, which is a return of the muscle fibers to their low tension-generating state.

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Introduction0:00–0:33

Even when you’re sitting perfectly still, when meditating for example, your muscles are still contracting a bit to stabilize joints and bones.
And this force that the muscles apply at rest is called muscle tone. On the other hand, when you pick up a 10 pound sack of potatoes, the force generated by the muscle contraction is much higher than the normal muscle tone in your biceps.
The pulling force transmitted through the muscle fiber is called the muscle tension. Now let’s dive into some basics of muscle physiology, starting with a single muscle cell or muscle fiber.

Physiology0:33–1:09

Within the muscle fiber is the sarcoplasm, which is the cytoplasm of a muscle fiber. The sarcoplasm is filled with stacks of long filaments called myofibrils.
And each myofibril consists of contractile proteins called thin actin and thick myosin filaments. These filaments don’t extend through the entire length of the muscle fiber - instead they’re arranged into shorter segments called sarcomeres.
Alright, now let’s zoom into a sarcomere. At the center of the sarcomere is the M line made of myomesin proteins, where the thick filaments attach.

Sarcomere1:09–2:37

At the borders of the sarcomere are the two Z-discs made of alpha actin proteins, where the thin filaments attach. For every thick filament, there are two thin filaments-one above and one below and the two types of filaments overlap.
The region with only thin filaments is called the I band and it appears light. The region with thick filaments is called an A band and it appears dark.
Now, most of the A band has overlap between the thick and thin filaments, but there’s an area towards the center called the H zone where there are only thick filament, so it appears slightly lighter.
When the muscle contracts, the thick filaments pull the thin filaments above and below it towards the M line. The Z discs attached to the thin filament also gets pulled towards the M line, and the whole sarcomere gets shorter.
Now, the A band does not change since it’s the length of the thick filament. But the H band and I band shortens because as the overlap increases, the region that consists of only thick or thin filament decreases.
At maximal contraction, there’s an almost complete overlap of the thick and thin filament and the H band and I band are almost completely gone.

Myosin2:37–3:03

Thick myosin filament is made up of hundreds of myosin proteins, each with a tail and two small club-like extensions, which are called myosin heads.
On the other hand, the thin filament actually looks like a pearl necklace that’s gently twisted. Each pearl represents one G-actin protein, which has an active site where the myosin head binds to during contraction.
Now, before myosin can bind actin, it first needs to power up. Part of the myosin head is an ATPase, meaning that it can cleave an ATP molecule to ADP and phosphate ion and release some energy.

Muscle contraction3:03–3:54

The energy is used to cock the myosin head backwards, into its high energy position. Next, the myosin head binds to the active site, and this is called cross-bridge formation.
Cross-bridge formation is the trigger to release of the stored energy in the myosin head, kind of like firing the catapult.
When that happens, the myosin head launches pulling the thin filament along with it. This is called the power stroke.
The combined power strokes of all the myosin heads lead to sliding of the thin filament along the thick filament, and this results in the contraction of the skeletal muscle.

Force of contraction factors3:54–4:12

The force of contraction depends on five factors: the size of the muscle fibers, the number of muscle fibers that are active during contraction, the frequency of stimulation, the length of the sarcomere, and the velocity of muscle shortening.
Let’s start with the number and size of the muscle fiber. First, the larger the muscle fibers is, the greater the number of myosin filaments and actin filaments within the sarcoplasm.

Size4:12–4:30

This means more cross bridges can form in large muscle fibers and this means a stronger contraction. Second, when we increase the number of muscle fibers, like during the Hulk’s transformation for example, we increase the force generated.

Number4:30–4:40

Frequency4:40–5:25

Third is the frequency of stimulation, also called the force-frequency relationship. Muscle contraction is initiated with an action potential which modifies receptors allowing calcium ions to flow from the sarcoplasmic reticulum into the sarcoplasm.
These calcium ions then bind to troponin regulatory proteins, which makes them change their shape and slide off of the active sites of thin filaments.
This allows myosin heads to bind to the actin. In order for a muscle to relax, calcium ions must be pumped back into the sarcoplasmic reticulum.
When the frequency of stimulation is increased, more calcium ions accumulate in the sarcoplasm, and the force of contraction increases.

Length5:25–6:39

The fourth factor on force of contraction is the length of the sarcomere, which is also termed as the length-tension relationship.
The relationship is directly proportional, meaning that the length of the sarcomere is directly related to the length of the muscle.
For example, let’s take the biceps brachii, a muscle in your upper arm. At rest, the length of the sarcomeres in the biceps brachii is around 2 micrometers.
At this length, the actin filament overlaps perfectly with myosin and this maximizes the number of crossbridges that they can form.
Now, if the biceps brachii contracts, the actin filaments are pulled towards the M line, meaning that the two actin filaments on either side of the M line start to overlap.
This overlap means some of the free sites on one actin filament will not able to bind to myosin, and fewer crossbridges means that there’s a weaker contraction.
On the flip side, straightening your arm stretches out the bicep and all of the sarcomeres get stretched as well. In this situation, the actin filaments slide away from the midline and the area of the actin-myosin overlap decreases meaning that there are fewer crossbridges and a weaker contraction.
And finally the fifth factor is the force-velocity relationship which relates the speed of the muscle’s shortening to the amount of force that it generates.

Velocity6:39–7:20

The quicker the muscle shortens the faster the actin filament is pulled towards the center of the sarcomere. Imagine the actin filament as the rope in a game of tug of war and the hands of the players are the myosin heads.
When the rope is moving slowly, more hands have time to grab onto it and a stronger force can be generated. When there’s a rapid contraction, the rope moves too fast, and fewer hands can grab on.
This means there’s less crossbridge formation and the result in a weaker contraction. Now a final distinction for muscle contractions is isotonic versus isometric contraction.

Isotonic vs isometric contraction7:20–8:22

In isotonic contraction, the muscle length changes but the tension within the muscle stays the same throughout the motion.
For example, in order to lift a load of a 10 pound bag of potatoes up to your apartment there must be at least 11 pounds of tension in the biceps brachii.
The moment the tension exceeds the load, your biceps brachii shortens, lifting the potatoes toward your shoulder. On the other hand, in isometric contractions, the length of the muscle stays the same, while the tension increases.
In isometric contraction, the tension within the muscle changes but the muscle length stays the same throughout. For example, if you try to lift a 1000 pound piano up to your apartment, then your biceps brachii will begin to build up more and more tension, but may never get to 1001 pounds of tension.
And because the load is so great, the biceps brachii doesn’t ever shorten - leaving you tired and without a piano. Alright, as a quick recap.

Review8:22–8:49

The force of contraction depends on five factors: the size of the muscle fibers, the number of muscle fibers that are active during contraction, the frequency of stimulation, the length of the sarcomere, and the velocity of muscle shortening.
Finally, in isotonic contractions, the tension stays the same while the muscle length changes, while in the isometric contractions, the tension changes, while the length stays the same.